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Toward Next-Generation Space Telescopes

When you think about what a space telescope goes through to get out of the gravity well and into its planned position, it’s a wonder that instruments this exacting can survive the journey. Launch vibration can reach six times Earth gravity and higher, while temperatures are all over the place as the launch vehicle moves from a temperate climate into the deep chill of space.

Countering all this while ever tightening the parameters of our instruments is a herculean challenge, but there is good news out of Goddard Space Flight Center, where Babak Saif and Lee Feinberg have gone to work on the problem. Working with Perry Greenfield at the Space Telescope Science Institute in Baltimore, the duo have been using an instrument called the High-Speed Interferometer (HSI), developed in Arizona by 4D Technology, to measure the changes that can occur over the surface of a spare 5-foot mirror segment, along with its support hardware, from the James Webb Space Telescope.

The HSI was designed to measure dynamic changes at the nanometer level in JWST optical components, but Saif and Feinberg have been probing even tighter requirements. When we get into the practice of studying exoplanet atmospheres, even atomic-scale distortion can affect a space observatory’s ability to focus and analyze the light of an Earth-class planet, which must be distinguished from the light of its parent star. To achieve this feat, the observatory would have to have optical components that move no more than 12 picometers, which is about one-tenth the size of a hydrogen atom. As demanding as JWST is, this ramps up the challenge.

Image: Goddard optics experts Babak Saif (left) and Lee Feinberg (right), with help from engineer Eli Griff-McMahon an employee of Genesis, have created an Ultra-Stable Thermal Vacuum system that they will use to make picometer-level measurements. Credits: NASA/W. Hrybyk.

No observatory yet built, including JWST, has been built that can meet such demanding requirements for stability. But the High-Speed Interferometer has allowed the GSFC scientists to measure dynamic changes across the mirror and structural components. What Feinberg and Saif bring to the table are new algorithms that can take the level of dynamic movement measurement down to 25 picometers, about twice what will eventually be required.

This is measurable progress toward the goal:

“These future missions will require an incredibly stable observatory,” said Azita Valinia, deputy Astrophysics Projects Division program manager. “This is one of the highest technology tall poles that future observatories of this caliber must overcome. The team’s success has shown that we are steadily whittling away at that particular obstacle.”

Working with 4D Technology, GSFC’s team has produced a ‘speckle interferometer’ that allows measurements on both reflective and diffuse surfaces at the picometer-level. The Goddard team is now analyzing its performance in a thermal vacuum test chamber that can control temperatures to the 1 millikelvin level. The plan moving forward is to test items within the chamber to see if the 12-picometer goal is within reach. “We’re getting there,” says Saif.

The key in this testing regime is to test at the same level across the entire structure of the telescope, not just movement between its mirrors. These HSI tests are encouraging, leveraging an instrument developed to work with the Webb telescope’s 18 mirror segments, mounts and supporting structures. Splitting light and recombining it to measure the effects of motion and vibration, the HSI allows the scientists to examine changes across the entire telescope.

The introduction of the speckle interferometer now points to the 12-picometer accuracy our future missions will demand. This is good news all around for atmospheric biosignature analysis.


{ 11 comments… add one }
  • John walker January 30, 2018, 20:47

    I really found this interesting. Thanks. Nice to hear of technical progress being made on important projects like LUVOIR. For those interested a recent paper described the technical roadmap toward this telescope. https://arxiv.org/pdf/1709.06660

  • xcalibur January 30, 2018, 22:41

    Another reason to expand into space is improving our astronomy. Ground-based telescopes must penetrate layers of distortion and noise in order to observe a signal. More space telescopes, and observatories on the far side of the moon, would help push back the frontiers of knowledge.

  • jonW January 31, 2018, 13:43

    I didn’t see a link to tell me more, so maybe someone could explain: is being able to measure so accurately the deformation that has occurred on the surface of a mirror enough to be able to compensate for the imaging aberration in software processing? In other words, do these measuring advances point to the way towards a corrective measure? Or are they “just” in service of being able to improve the manufacturing process?

    I’m also confused: I thought JWST *was* going to be able to image exoplanets and do spectral analysis of their atmosphere, but this post suggests that since it won’t have the 12-picometre stability required, it won’t be able to perform these observations.

    And a last question! The measuring technology allows the team to pin down structural changes down to (currently) 25 picometres–but without the rigours of a launch, the turbulent flight through the atmosphere and the exposure to space, how can they extrapolate to real-world (“real-space”?) conditions and know how the mirrors will actually hold up?

    • Paul Gilster January 31, 2018, 15:00

      Jon, JWST should be able to give us some interesting atmospheric data, but only when using transmission spectroscopy, where a transit across a red dwarf is needed to use the technique. What the 12-picometer stability refers to is when we’re actually looking at a separate planet as a target — no transit, but a separate pixel out there that we can investigate directly. As to the launch rigors, these can be tested at ground facilities, as payloads commonly are. I’ll have to pass on the first question, though I’ll see if I can find somebody to answer it.

      • Michael Fidler January 31, 2018, 22:24

        As to your first question, the original photons coming from the object thru the telescope is like your eyes and software processing is like a TV, the best possible image is the most important part of a telescope in space or on the ground.
        The telescope forms an image of a star called an Airy Disc; “In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk is of importance in physics, optics, and astronomy. The diffraction pattern resulting from a uniformly-illuminated circular aperture has a bright region in the center, known as the Airy disk, which together with the series of concentric bright rings around is called the Airy pattern.

        The most important application of this concept is in cameras and telescopes. Due to diffraction, the smallest point to which a lens or mirror can focus a beam of light is the size of the Airy disk. Even if one were able to make a perfect lens, there is still a limit to the resolution of an image created by such a lens. An optical system in which the resolution is no longer limited by imperfections in the lenses but only by diffraction is said to be diffraction limited.

        A very good explanation on the issue of telescope resolution;


        Things that affect the Airy pattern; imperfections in the mirrors surface, secondary mirrors size ratio to main mirror and the secondaries spider, cause the rings to brighten and lower the contrast and resolution of a telescope. One of the goals of astronomers has been to produce a perfect telescope and this is why they have been working on adaptive optics to correct the image smearing caused by the atmosphere.
        One area that has not been addressed is the problems with secondary mirrors degrading the image. One way that can be corrected is by putting a mirror segment ahead of the secondary mirror to fill in the obstruction caused by the secondary. This would then be added to the image by focusing it thru a small aperture in the secondary and focus it into the same phase as the rest the the segments. I have read were this can improve the resolution and contrast of the main mirror to twice it size!

    • John walker January 31, 2018, 15:52

      Hi Jon. The goal is to produce a sharper raw image. This will assist in direct imaging of exo planets close to their stars using a coronagraph in the instrument. As Paul emphasised a capacity to directly image a planet greatly increases the numbers of objects available for spectroscopic analysis since only about 1 in 200 systems are edge on to the Earth allowing for transit studies. JWST doesn’t have a coronagraph so direct observations will not be possible. WFIRST will but at 2.4m it’s a pretty small instrument. Internal coronagraphs generally steal a lot of light so a large aperture is very useful. Like that of LUVOIR.which is planned to be perhaps 12m.

  • PW February 2, 2018, 12:05

    I still think the moon is the place to build big telescopes. Build one in the big crater at the lunar south pole. Cryogenic conditions to receive the faintest light. Ability to get hands on to fix problems. Opportunity to make big leaps in engineering and astronomy at same time.

  • ljk February 13, 2018, 16:52

    WFIRST is in trouble (no) thanks to the new federal budget:


    What Would It Mean for Astronomers If the WFIRST Space Telescope Is Killed?


  • ljk February 16, 2018, 13:37

    AAS Officials Concerned with Proposed Cancellation of WFIRST


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